13

According to some sources, the purpose of MCAS on Boeing's 737 MAX variants is to increase the back-force needed to further raise the nose when flying manually at high angles of attack, in order to give the airplane acceptable handling characteristics when approaching a stall.

Both the MAX variants and their predecessors, however, already have an elevator Feel and Centering Unit (page 8), the purpose of which appears to be the generation of appropriate stick force feedback in all stages of flight. If so, then, at first sight, this would be the appropriate unit in which to implement the function of MCAS, raising the question of why MCAS would be the preferred solution.

A few possibilities have occurred to me, but they are just guesses:

  1. MCAS functionality needs angle-of-attack input, which may not be available where the Feel and Centering Unit is located (in the tail), and it would be complicated to get that information to it.

  2. This answer states the the Feel and Centering Unit is a mechanical computer; given that, it therefore might not be easily modified.

  3. Modifying the Feel and Centering Unit would require re-certifying it in toto, not just its new feature.

  4. In addition to modifying the handling characteristics, MCAS is also seen as contributing to stall prevention directly, by reducing the angle of attack.

NB: Recently, Dominic Gates wrote an informative article in the Seattle Times about the origins of MCAS.

sdenham
  • 1,348
  • 8
  • 27

4 Answers4

12

On the 737 NG, at high angles of attack, the nose of the plane would naturally pitch down, helping to recover from a stall and to increase airspeed.

The larger engine nacelles on the 737 MAX are located forward of the center of gravity, which means that at high angles of attack they are pushing the nose up. MCAS helps to push the nose down in this situation, similar to how the 737 NG would behave.

The added nose down trim has the side effect of requiring more elevator input at high angles of attack, but that was not the primary purpose.

There are several reasons that the Feel and Centering Unit would have a difficult time providing similar functionality:

  1. The airplane should pitch down even the absence of control input, so it could not be done by simply changing the way the elevators respond to pilot input.
  2. MCAS takes input from angle of attack, altitude, flap position, and airspeed. The Feel and Centering Unit currently just senses airspeed, so it would somehow need to get the other inputs.
  3. The Feel and Centering Unit is a mechanical computer, so adding new inputs and changing the behavior could be very complex to design and certify.

Another possibility would be to use the Mach Trim system to adjust the Feel and Centering Unit. The Mach Trim system uses the flight computer to adjust the elevator neutral position to provide stability at higher speeds. While the flight computer should have all of the information needed, stabilizer trim provides much more control authority.

abelenky
  • 30,696
  • 9
  • 92
  • 143
fooot
  • 72,860
  • 23
  • 237
  • 426
  • Are you saying the the handling characteristics issue that the Leeham News article discusses is not the reason for MCAS? Is the pitch-down behavior of the NG prior to the stall? – sdenham Apr 05 '19 at 18:55
  • He’s not saying that the article is wrong. The handling characteristics at high AoA is precisely what MCAS is for. The FCU already increased aft stick forces when the stall warning was active in the NG. The difference is that, at a certain AoA the lift created by the engine nacelles became a factor, making a stronger nose up moment than the NG created at the same AoA. One selling point of the MAX is that it required very little transitional training for pilots currently flying NGs. They used the MCAS to more closely match the performance of the NG. – TomMcW Apr 06 '19 at 22:52
  • 1
    @TomMcW I don't disagree with the points you are making, but my question is not "what is MCAS for?" – sdenham Apr 08 '19 at 01:43
  • I suppose the reason for the instability of pushing nose at high angle of attack is the center of lift is in front of the center of mass. But why does shifting the engine forward shift the center of lift? Without considering the secondary effect, the thrust of the engine is directed parallel to the fuselage and thus the wing. So there should not be a discernible difference in the torque with respect to the center of mass of the whole plane. – Hans Apr 17 '19 at 02:18
  • @Hans at high AOA the engines are producing aerodynamic lift. Thus the further forward (or larger) they are, the further that moves the total center of lift forward. – fooot Apr 17 '19 at 02:26
  • Your claim puzzles me in two parts. First, the engine itself produces lift in addition to the thrust parallel to the fuselage and wing. Second the lift thus produced is dependent on the AOA. Could you please elucidate those points or provide some references, best if they come with mathematical derivation? – Hans Apr 17 '19 at 03:15
  • @Hans All engine nacelles produce lift as a consequence of their approximately cylindrical shape. When the airflow is parallel to the axis of a cylinder, no lift is produced, but with the cylinder axis at some angle of attack, some lift will be created. As is generally the case, this lift is some function of that angle, the airspeed and density. Because the engines are forward of the wing, this lift is destabilizing. For low drag, the engines are mounted so as to have a small angle of attack when cruising, so this lift is only significant at lower speeds and higher AofA... – sdenham Apr 18 '19 at 15:56
  • @Hans To some extent, this affects all airplanes with engines mounted under / forward of the wing, but in the case of the 737 MAX, this has become a problem because the engines have become much larger and more forward than the ones the wing and stabilizer were originally designed in concert with. https://leehamnews.com/2018/11/14/boeings-automatic-trim-for-the-737-max-was-not-disclosed-to-the-pilots/ NB: lengthening the fuselage also destabilizes because a cylinder's center of pressure is forward of its geometric center. https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stability-part-2/ – sdenham Apr 18 '19 at 16:12
6

A change in the pitch feel system wouldn't solve the problem. It's the MAX's natural behaviour separate from the flight control system (that is, behaviour when you aren't touching the controls). As Fooot says, the MAX's engines have the effect of moving the overall center of lift forward somewhat, which is more or less the same thing as moving the center of gravity aft.

The airplane, in certain regimes (flaps up), ends up being neutrally or almost neutrally stable in pitch, especially at higher power settings where the thrust is contributing to the nose-up moment - bad enough that the airplane's pitch attitude would drift up when it should be rock solid, and worse, the natural pitch down you should get with a speed decrease wasn't there or was very weak. It could be countered by the pilot, but the hand flying workload goes way up when you have to constantly intervene with an airplane that has a bit of a mind of its own. Flying just about any airplane with an excessively aft CG is like that.

The proper fix would be to move the operating center of gravity range forward to cancel out the influence of the more forward engines and make the horizontal tail larger to compensate so that the tail power required to rotate at takeoff (which usually the tail's hardest job) is still there. They didn't want to go that route and decided to use software to run the stab in the background to "mask" the stability problem from the pilot so that they could keep the C of G range where it was. It's basically an artificial stability system with a narrow operating requirement added on as a band-aid to avoid a far more expensive modification.

It's not the first time it's been done. I recall something similar was done on another type, namely the MD-11, that allowed the airplane to be operated with a farther aft CG than normal, to reduce tail down force in cruise, reducing trim drag. I dimly remember an incident from long ago where the system was disconnected while in cruise and the pilot took over hand flying his neutrally stable airliner cruising at mach point whatever, and a Pilot Induced Oscillation got started that rattled people in the back around pretty good, like shaking a tube of Pringles potato chips.

UnrecognizedFallingObject
  • 13,046
  • 3
  • 38
  • 111
John K
  • 130,987
  • 11
  • 286
  • 467
  • This answer does not mention the feel/centering unit, but implicitly you seem to be saying that it would not be a candidate solution because, contrary to the Leehams News article, satisfying the handling characteristics requirement was not the problem that MCAS was developed to solve. If so, could you modify your answer to make this point explicitly, preferably with references to an authoritative source on what made some sort of mitigation necessary? – sdenham Apr 05 '19 at 20:10
  • @sdenham, the problem with the centering unit is that it is designed to pull the yoke towards the center, but to augment stability you need to move the center around, which is what trim is for. – Jan Hudec Apr 06 '19 at 08:00
  • 1
    @JanHudec I am not suggesting that the Feel and Centering Unit, as currently implemented, solves the problem - if it did, there would be no need for any sort of modification to compensate for the effect of the new engines. – sdenham Apr 06 '19 at 13:48
  • @sdenham, and I am not talking about current implementation, but overall purpose. The problem that MCAS was developed to solve is poor handling characteristics, but modifying handling characteristics is a problem completely unrelated to the purpose of the feel and centering unit. – Jan Hudec Apr 06 '19 at 19:02
  • 1
    @JanHudec According to https://www.737ng.co.uk/B_NG-Flight_Controls.pdf "The elevator feel computer provides simulated aerodynamic forces... Feel is transmitted to the control columns by the elevator feel and centering unit." This (and the name) suggests that it does more than centering, and modifying the control forces is what the Leeham News article says is the purpose of MCAS. – sdenham Apr 06 '19 at 20:02
  • @sdenham: “modifying control forces” is too broad a term. Feel and centering unit provides “simulated aerodynamic forces”, but the aerodynamic forces always push the elevator towards the center (relative to the movable stabilizer), just increasing with dynamic pressure, so it still is “just centering”, with variable force. Changing the position of the center is why the stabilizer is movable, so adding logic that moves it is the more obvious change. There also were some other cases of auto-trim already since the previous model. – Jan Hudec Apr 06 '19 at 20:38
  • Feel and centering units are just boxes with bungee springs inside, and a mechanical system that has the springs work against a lever with a variable fulcrum. The spring pack will be driven by a lever linked into the control circuit. When the pilot moves the controls he is compressing the bungee springs. The variable fulcrum is moved by a computer that adjusts it to change the force felt by the pilot, which simulates the increase and decrease in control forces with speed and other factors.The surfaces are driven by irreversible hydraulics so there is zero aerodynamic feedback to the controls. – John K Apr 07 '19 at 00:47
  • @JanHudec According to Bjorn Ferhm, the MAX does not become statically unstable, and the specific issue is that "the aircraft OEM is asked to propose fixes to the pitch characteristics of the aircraft so it can be judged acceptable for use by a minimum standard (re. training and proficiency) pilot", and it was judged unacceptable (prior to MCAS) because, when approaching the stall, " the pilot would feel as if the aircraft suddenly pitches up faster for the same stick input." You may disagree with Bjorn, but if it is so, the issue is precisely the size of the force towards the center position. – sdenham Apr 07 '19 at 02:06
  • @JohnK Thank you for your detailed comments. Is it unreasonable to say that, at least in principle, the variable fulcrum could be adjusted to correct the problematic 'lightening' of the controls at high angles of attack, by multiplying the restoring force by some factor > 1.0? I am wondering if the computer that adjusts the fulcrum is a simple mechanical (or hydro- or electro-mechanical) analog device, which I am guessing would be hard to add AofA input and additional rules to, and might have to be completely re-certified for all flight regimes? – sdenham Apr 07 '19 at 02:30
  • @JohnK With zero aerodynamic feedback (and thus with the stick force being entirely set by the feel/centering unit), the unit's job seems to be 1) have the stick free/neutral position correspond to the elevator being in line with the stabilizer 2) stick force is always towards neutral 3) at a given airspeed, a given change in force results in a fixed change in pitch 4) increase force with airspeed. Given theres no feedback, the reduced stability would appear as a large change in pitch for a small displacement change, which would feel 'light' because the force is proportional to displacement. – sdenham Apr 07 '19 at 02:58
  • The root issue is the behavioural problem is not something that can be addressed by changes to the force regime felt by the pilot. The issue is "external" natural behaviour you might say, that required "external" intervention by a system that operates in the background and forces the airplane to behave, in a way that is transparent to the pilot. Using the stab screw jack is the easiest way to do it. An alternative could be a device that works sort of like a yaw damper, moving the elevator downstream of the feel system so the pilot is not aware of it, but that would be way more expensive. – John K Apr 07 '19 at 04:01
  • @sdenham, I did not say it is statically unstable, but what you describe sounds exactly like there is a region of insufficient static stability just before stall. As the angle of attack increases, the centre of pressure—on every aircraft—moves slightly forward, creating pitch up moment and reducing static stability. If that's indeed the effect that became too strong here, it requires creating a pitch-down moment, i.e. trimming nose down. Without that the pilots can simply trim for the dangerously slow speed and multiplying the force won't help, because it will be zero in the first place… – Jan Hudec Apr 07 '19 at 20:29
  • @JanHudec You will see stated earlier in the same article, reduced stability at high angles of attack is indeed the cause of the unacceptable handling characteristics. While your issue of pilots being able to trim out the restoring force seems valid, it would also seem something you could say about any airplane approaching a stall. It is also true that pilots can use the trim switches to countermand ACAS, so ACAS does not eliminate the risk. (continued) – sdenham Apr 08 '19 at 00:45
  • @JanHudec When you say "it requires creating a pitch-down moment", you seem to be saying that ACAS is intended to be a sort of pre-stall stick pusher? If you think that is the purpose of ACAS (something that the feel & centering unit could not do), perhaps you could write it up as an answer. – sdenham Apr 08 '19 at 00:48
  • The problem as I understand it was that the airplane when at low speed flaps up could start to pitch up and slow down without any pilot input. It would drift below its trim speed in other words. MCAS applies a subtle correction by running the stab nose down about 9 sec, then stopping and waiting to see what happens. If airplane is still pitching up it repeats the process. I don't believe the '37 even has a pusher. – John K Apr 08 '19 at 00:57
  • @JohnK What you are saying here is that it becomes statically unstable at high angles of attack. I have seen some sources (or opinions) saying that, but others (including Bjorn Fehrm's article) saying no, it is just reduced stability. This is just one issue surrounding MCAS where there are more opinions in circulation than facts, and maybe I should first ask about them... If 737s do not have a stick pusher, that might be an argument for MCAS, as MCAS has pusher-like behavior. – sdenham Apr 08 '19 at 01:22
  • @JohnK Let me rethink this somewhat... airplanes will continue to slow down (and consequently have an increasing angle of attack) if they are initially trimmed, on the back of the drag curve, are disturbed in such a way as to decrease the speed momentarily, and no corrective action is taken. This is not considered to be unacceptable behavior, at least in general aviation (I'm not sure where this observation is going...) – sdenham Apr 08 '19 at 01:41
  • @JohnK I found another source (though not primary) saying the issue is reduced force, not full instability: "As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics".' Also '[MCAS] is not for stall prevention (although indirectly it helps)' http://www.b737.org.uk/mcas.htm ... I have also seen FAR §25.173 (stability). – sdenham Apr 09 '19 at 01:15
  • You support @fooot's opinion and claim that "the MAX's engines have the effect of moving the overall center of lift forward". I doubt that and have posed it to him below his answer: "Your claim puzzles me in two parts. First, the engine itself produces lift in addition to the thrust parallel to the fuselage and wing. Second the lift thus produced is dependent on the AOA. Could you please elucidate those points or provide some references, best if they come with mathematical derivation?". However, he is unable to answer it. Do you have an answer to my question? – Hans Apr 17 '19 at 17:36
  • @JohnK -- the McD aircraft you're after for the pitch stability issues is the MD-11 aka the "Pitch B#$!@ of Long Beach", BTW – UnrecognizedFallingObject Jun 05 '19 at 11:47
0

Boeing wanted the MCAS effect to be transparent to the pilot, as a proof nothing about it was mentioned in the FCOM.

Acting on the feel and centering mechanism would require a sudden effect on the column which would have been noticed and declared as a fault by the pilots since to get the same effect as does the trim, considering the elevators area compared to the THS area, you would need a sudden tremendous visible displacement of the column centering while a single trim shot of 2.5° ( .6° in the original design) is less disturbing, mainly because short trim movements are possible for other reasons even though in manual flight, such as Mach trim.

Who will bother much about a single trim shot, was it not a faulty AOA producing repetitive trim shots? As a matter of fact it remained transparent till the faulty AOA producing the crashes

user40476
  • 1,772
  • 9
  • 27
  • Thank you for replying, but could you expand on why it would inevitably result in a sudden effect? Would it be impossible for the unit to progressively increase the force, relative to that produced by the NG's unit, as the airspeed decays towards the stall? (I say speed rather than AofA, because, AFAIK, the unit does not get that data, but it does have a pair of dedicated pitots.) – sdenham Jun 04 '19 at 14:06
  • @sdenham, Increasing the force makes a difference with the NG thus the need for training but Boeing didn’t want extra simulator training, thus the need of MCAS and the most transparent possible. It is sudden because it occurs unexpectedly at flap retraction once speed is above 230 knots. Please refer to the following website. https://www.reuters.com/article/us-ethiopia-airplane-regulator-insight/regulators-knew-before-crashes-that-737-max-trim-control-was-confusing-in-some-conditions-document-idUSKCN1RA0DP – user40476 Jun 04 '19 at 17:15
  • How would using the feel and centering unit be seen as a fault by pilots, but using trim wouldn't be? – fooot Jun 26 '19 at 16:29
0

The root of the problem with the MCAS is that the Angle of Attack sensor is near the nose instead of on the leading edge of the wing, where it belongs. It's supposed to be measuring the Angle of Attack OF THE WING. Instead, it measures the Angle of Attack OF THE NOSE. These measurements are not the same while the aircraft changes pitch.

With the long fuselage, bringing the nose down from climb to cruise will falsely indicate a nose-up attitude because the fuselage is rotating around the wing. This is why both crashes occurred from problems that arose when changing from climb to cruise. Whether the MCAS malfunctions or not depends completely on the rate at which the pilot changes pitch. Push forward too far on the stick, and you are doomed, when the MCAS takes over pitch control and will not give it back.

The false Nose Up reading moves the elevator trim to further lower the nose, again triggering a false nose-up reading. It's a feedback loop that will not stop until MCAS hits its limit, which gets reset every time the pilot follows instructions and presses the reset button. Press it more than three times, and trim is set to the mechanical limits of the elevator trim. In the second crash, the pilot hit the reset button more than 20 times.

Will Martin
  • 218
  • 2
  • 4
  • But near the wing you have side effects from the flow around the wing. There are plenty of alpha vanes on fuselages, e.g. on all airbus aircraft that I know of. It would also be possible to correct that error mathematically with the pitch rate and airspeed. If you move the angle of attack sensor to the wing you'd have to calibrate it for all different flap settings, with and without failures of either flaps or slats. Much easier to just calibrate it on the fuselage where at least the geometry doesn't change. – Jan Jun 26 '19 at 16:22